Method and apparatus for measuring the thickness of one or more layers of a multi-layer film obtained by blow extrusion process

Abstract

A method is described for measuring, in a multi-layer film obtained by blow extrusion process and formed by one or more layers of a first material and a second material, the total thickness of the first material and/or the second material. The method includes the steps of: a) acquiring by means of an X-ray sensor a first measurement signal representative of the total thickness of the film; b) acquiring by means of a capacitive sensor a second measurement signal which is the sum of the signals given by the first and second materials of the film. The signal given by each material of the film is a function of the thickness of the material; and c) calculating, from the first and second signals, the total thickness (L1 of the first material and/or the total thickness (L2) of the second material.

1 . A method for measuring, during production by blow extrusion process of a multi-layer film formed by one or more layers of a first material and one or more layers of a second material, the total thickness (L 1 ) of the first material and/or the total thickness (L 2 ) of the second material, comprising:
 (a) acquiring by means of an electromagnetic radiation sensor a first measurement signal (S x ) representative of the total thickness of the film;   (b) acquiring by means of a capacitive sensor a second measuring signal (S cap ) which is the sum of the signals given by the first and the second material of the film, wherein the signal given by each material of the film is a function of the thickness (L 1 , L 2 ) of that material; and   (c) calculating, from said first and second measurement signals (S x , S cap ), the total thickness (L 1 ) of the first material and/or the total thickness (L 2 ) of the second material.   
     
     
         2 . The method according to  claim 1 , wherein said electromagnetic radiation sensor is an X-ray retro-reflection sensor. 
     
     
         3 . The method according to  claim 1 , wherein said steps a) and b) of acquiring a first measurement signal (S x ) by means of an electromagnetic radiation sensor and acquiring a second measurement signal (S cap ) by means of a capacitive sensor, are performed with said sensors arranged facing an outer surface of the bubble (B). 
     
     
         4 . The method according to  claim 3 , wherein said steps a) and b) of acquiring a first measurement signal (S x ) by means of an electromagnetic radiation sensor and acquiring a second measurement signal (S cap ) by means of a capacitive sensor, respectively, are performed with said sensors arranged above each other and at a certain distance from each other, in a same plane oriented tangentially with respect to an outer surface of the bubble (B). 
     
     
         5 . The method according to  claim 4 , further comprising the step of measuring the distance of said plane from the outer surface of the bubble (B) by means of an ultrasonic sensor. 
     
     
         6 . The method according to  claim 2 , wherein said steps a) and b) of acquiring a first measurement signal (S x ) by means of an electromagnetic radiation sensor and acquiring a second measurement signal (S cap ) by means of a capacitive sensor, respectively, are performed by moving said sensors along a circular path around the bubble (B). 
     
     
         7 . The method according to  claim 1 , wherein said step c) of calculating the total thickness (L 1 ) of the first material and/or the total thickness (L 2 ) of the second material is based on solving the following system of equations: 
       
         
           
             
               
                 
                   S 
                   X 
                 
                 = 
                 
                   
                     k 
                     X 
                   
                   · 
                   
                     ( 
                     
                       
                         L 
                         1 
                       
                       + 
                       
                         L 
                         2 
                       
                     
                     ) 
                   
                 
               
               ⁢ 
               
 
               
                 
                   
                     S 
                     cap 
                   
                   = 
                   
                     
                       
                         k 
                         1 
                       
                       · 
                       
                         L 
                         1 
                       
                     
                     + 
                     
                       
                         k 
                         2 
                       
                       · 
                       
                         L 
                         2 
                       
                     
                   
                 
                 , 
               
             
           
         
         wherein the parameter k x  is continuously derived from data supplied by dosing means of the film production plant, which measure the quantities of the first material and the second material fed into the plant, 
         wherein the parameter k 1  is determined, during the start-up phase of the film production plant, based on the value of said second measurement signal (S cap ) when the film (F) is formed by the first material only, and 
         wherein the parameter k 2  is determined during the film production cycle, based on the average value of said second measurement signal (S cap ) and the average values of the total thickness (L 1 ) of the first material and the total thickness (L 2 ) of the second material provided by said dosing means, from the following equation: 
       
       
         
           
             
               
                 k 
                 2 
               
               = 
               
                 
                   ( 
                   
                     
                       
                         S 
                         cap 
                       
                       _ 
                     
                     - 
                     
                       
                         k 
                         1 
                       
                       · 
                       
                         
                           L 
                           1 
                         
                         _ 
                       
                     
                   
                   ) 
                 
                 / 
                 
                   
                     
                       L 
                       2 
                     
                     _ 
                   
                   . 
                 
               
             
           
         
       
     
     
         8 . An apparatus for measuring, in a multi-layer film obtained by blow extrusion process and formed of one or more layers of a first material and one or more layers of a second material, the total thickness (L 1 ) of the first material and/or the total thickness (L 2 ) of the second material, said apparatus comprising:
 an electromagnetic radiation sensor arranged to provide a first measurement signal (S x ) representative of the total thickness of the film;   a capacitive sensor arranged to provide a second measurement signal (S cap ) which is the sum of the signals given by the first and second materials of the film, wherein the signal given by each material of the film is a function of the thickness (L 1 , L 2 ) of said material; and   processing means for calculating, from said first and second measurement signals (S x , S cap ), the total thickness (L 1 ) of the first material and/or the total thickness (L 2 ) of the second material.   
     
     
         9 . The apparatus according to  claim 8 , wherein said electromagnetic radiation sensor is an X-ray retro-reflection sensor. 
     
     
         10 . A blown film extrusion plant for producing a multi-layer film, comprising an apparatus according to  claim 8 . 
     
     
         11 . The plant according to  claim 10 , wherein said apparatus further comprises a measuring head on which said electromagnetic radiation sensor and said capacitive sensor are mounted so as to lie in a same plane oriented tangentially with respect to the outer surface of the bubble (B), guide means arranged around said bubble (B), coaxially to said bubble (B), and on which said measuring head is movably mounted along a circular path, and driving means for controlling the movement of said measuring head on said guide means. 
     
     
         12 . The plant according to  claim 11 , wherein said apparatus further comprises an ultrasonic sensor arranged to measure the distance between the plane on which said electromagnetic radiation sensor and said capacitive sensor lie and the outer surface of the bubble (B).